When the contrast of the display is set very high and the backlight is reduced to lower values, just for testing, the screen flickers. Outside the room with bright daylight it is o.k.
Some guys say this is a normal effect.

Normal. The scanning frequency if the LCD and the PWM frequency of the backlight are obviously close or causing interference. On the other hand, you do not see the text if you set the contrast to ridiculous levels

Since I am very sensitive to flickering I have the backlight always at 100%. I also hate the crappy LED braking lights on modern cars, the PWM is also way too low....